1. Field of the Invention
This invention relates to a golf ball injection mold and a method for injection molding golf balls using the mold.
2. Prior Art
The methods in the prior art for molding golf balls using a mold, especially the methods for preparing golf balls consisting of a core and a cover by molding the cover around the core using a mold include injection molding and heat compression molding.
Of these, the injection molding method is widely utilized because formation of fins is minimized and the entire process is simple in that extra steps of preforming half cups and placing half cups around the core as required in the heat compression molding method are unnecessary. The injection molding method is described by referring to the manufacture of two-piece solid golf balls. FIG. 7 and FIG. 8 are schematic cross-sectional views of a prior art injection mold. As shown in FIG. 7, a cover-molding mold 1 includes an upper mold section 1a and a lower mold section 1b, which are mated along a parting plane P to define a hollow spherical cavity 2 therein. The cavity-defining surfaces of the mold sections are provided with dimple-forming protrusions (not shown). A solid core 3 is placed in the cavity 2 as an insert. The core 3 is vertically supported by a plurality of vertically extending support pins 4 so that the core is positioned at the center of the cavity 2. A space 2a is defined between the core 3 and the cavity-defining surface. In this state, a cover stock based on thermoplastic resin is melted in an injection cylinder of an injection molding machine (not shown). A cover stock injection path is shown in FIGS. 8A and 8B as including a main runner 12 connected to the injection cylinder, an annular cold runner 11 connected to the runner 12, and a plurality of nozzles 10 disposed in a cavity surrounding portion 13 and equidistantly spaced on the inner circumferential side of the annular runner 11, each nozzle 10 having a gate 9 in communication with the cavity 2. The molten cover stock is introduced to the annular cold runner 11 through the main runner 12, and then injected into the space 2a through the gates 9 of the nozzles 10. Immediately before the injection of the cover molding material 5 is completed or at the same time as the completion of injection, the support pins 4 are withdrawn until their distal ends are flush with the cavity-defining surface. In this way, the core 3 is enclosed with a cover having a multiplicity of dimples. After cooling, the upper mold section 1a is opened and the molded golf ball is released from and taken out of the lower mold section 1b.
It is noted that in FIG. 7, a pin 7 is fixedly received in a hole 6 to define a venting space 8 at each of opposite poles. During molding, air in the mold cavity 2 is displaced outside through the venting spaces 8.
As shown in FIGS. 8A and 8B, the gate 9 is joined to the cavity 2 such that the angle of connection of the gate to the cavity surface 2' is perpendicular as measured relative to a tangent to the cavity surface 2' at the connection. The cover molding material is then injected toward the center C of the core 3. Since the core 3 is supported by the support only pins 4, there is a possibility that the support will not withstand the injection pressure of the cover molding material from the gates 9, and the core 3 will be off-centered or compression deformation is yielded at the portions of receiving the injection pressure on the core. Particularly when a thin gage cover is injection molded over a large diameter core, the space between the core and the cavity-defining surface is fairly narrow and thus requires a higher injection pressure than usual. As a result, a higher injection pressure is applied to the core, with a more likelihood of the core being off-centered. Then the molded golf ball is reduced in symmetry so that the ball may differ in distance or trajectory depending on the position where it is hit, failing to acquire stable flight performance.